Scope of this study

Enzymes are used for different industrial applications, such as detergents, textile and food industry. However, use of enzymes in the oil and gas industry has been suggested recently.

Feng et al. (2007) reported increased oil production by injection of modified enzyme solutions in both laboratory core floods and a field scale pilot test. However, data on the possible use of enzymes to increase recovery after waterflooding is very limited. Further, to our knowledge, there have been no comprehensive attempts at investigating the mechanisms by which enzymes contribute to enhanced oil recovery. The scope of the present work is thus to study how enzymes can affect oil recovery, and further to investigate the mechanism by which enzymes contribute to more incremental oil.

The experimental work started by examining the effect of enzymes on water-solid and oil-water interactions as inferred from interfacial tension, electrophoretic mobility and wettability measurements (Chapter 7 and 8). The measurements were done with different types of enzyme added to the brine phase and compared with samples without enzyme. Following this initial phase, the effect of enzyme solutions on enhanced oil recovery from sandstone and carbonate rocks was tested (Chapter 9). Finally, and attempt to link the static and the dynamic measurements were made by studying the displacement process with and without added enzyme in micromodel experiments (Chapter 10).

Chapter 11. Summary of the main results

11.2 What can enzymes do?

Enzymes are a specific group of proteins that catalyse many thousands of biochemical reactions by lowering the activation energy and thus dramatically accelerating the rate of the reaction. The question here is how using enzymes can lead to enhanced oil recovery?

Crude oils contain a large number of polar molecules such as phenols, carboxylic acids, sulphur and nitrogen. These polar functional groups can provide acidic, basic or even zwitterionic sites at the interface between crude oil and water and thus have direct impact on fluid/rock interactions related to wettability and fluid/fluid interactions related to interfacial tension (IFT). Wettability and IFT have a strong influence on the distribution and flow of fluids in the reservoir rock. Together with the structure of the porous medium they are the major parameters affecting the amount and distribution of capillary trapped oil in a reservoir after waterflooding. More details of these oil trapping mechanisms can be found in chapter 2.

Enzymes can contribute to changing the fluid-rock and/or fluid-fluid interactions mainly by catalysing the break down of crude oil components. As an example, hydrolase enzymes catalyse bond cleavage by introduction of water which may break these molecules down into (i) smaller molecules with increased water solubility and reduced interfacial activity and/or (ii) more polar molecules (e.g. ester hydrolysis to form acid + alcohol). These compositional changes may influence on wettability and interfacial tension. Besides, as discussed in detail in chapter 6, proteins are known to adsorb onto solid/liquid and liquid/liquid interfaces. The protein nature of the enzymes may thus give rise to changes in the interaction energy between crude oil, brine and rock.

11.3 “Evidence” of wettability alteration by enzymes

Adhesion tests and contact angle measurements were performed to evaluate the ability of enzymes to change the wettability of silica glass surfaces. Different types of enzyme from different classes, hydorlases, protases, esterases, as well as a pure protein, were used in the experiments. The results of all contact angle and adhesion measurements were presented and discussed in detail in chapter 7. Using the adhesion method, it was shown that adding enzymes to the brine solution can change the adhesion behavior of the crude oil on the solid surface from adhesion to non-adhesion.

Chapter 11. Summary of the main results

Contact angle measurements showed that enzyme-proteins affect the wettability of glass surfaces for both untreated glass and glass aged in crude oils at elevated temperature (80^C).

All enzymes changed the wettability of the glass surfaces towards a more water-wet condition. Among the tested enzymes, protease enzymes was found to have the least effect while esterase enzymes had the most pronounced effect on the oil-brine-quartz contact angle.

The latter changed contact angle independent of their concentration and all concentrations showed almost the same change in contact angle. Adding Greenzyme and pure protein also had significant effect on contact angle, comparable with the results obtained with esterase enzymes. The results showed that the wettability alteration of the glass surface appears to be most pronounced when we have a solid surface with more intermediate wettability i.e. contact angle between 65 and 100 degrees.

The mechanisms underlying contact angle change by enzymes can be discussed with two perspectives. As enzymes they catalyze the breakdown of certain bonds which may exist in the crude oil, and thereby change the interaction forces at the crude oil/brine/glass interface.

However, the protein nature of enzymes should also be considered.

Esterase enzymes catalyze the hydrolysis of ester bonds to form acid and alcohol.

Dissociation of the acid may lead to increasing repulsive electrostatic forces at interfaces of crude oil/brine/glass surface (see Figure 11.1).

Chapter 11. Summary of the main results

More electrostatic repulsion due to breaking of ester bonds

Figure 11.1: Breaking down of ester bonds by NZ group enzymes and alter the wetting behavior of the solid.

Gas chromatography measurements on mineral oil with added ester showed the breakdown of ester bonds and a significant decrease in amount of ester in the oil. This phenomenon was also investigated measuring the electrophoretic mobility of oil droplets dispersed in brine in the presence of enzymes (see chapter 8). The electrophoretic mobility is directly proportional to the magnitude of the charge on the oil droplets in the brine solution.

Enzymes affect the electrophoretic mobility of the oils in brine as a result of change in the electrostatic forces at oil-water interface. Adding Greenzyme to the brine cause an increase in the electrophoretic mobility (more negative). Although the two crude oils and the mineral oil had different electrophoretic mobilities initially, the values are almost identical when GZ was added to the brine phase, meaning that adding GZ into the solution overshadowed the effect of other substances. The reason can be due to added stabilizers (surfactants) to the Greenzyme which have the negative charge (Greenzyme material safety data sheets). Esterase enzymes seemed to have limited effect on the electrophoretic mobility of crude oil A, the mobility was decreased for crude oil B and mineral oil with added ester. This can be explained by the amount of acid and base functional group in crude oil B which is more compare to crude oil A and existence of ester in the mineral oil that cause more electrical charges at the oil-water interface and thereby more interaction of enzyme at the interface.

Chapter 11. Summary of the main results

Adsorption of the protein at the crude oil-brine-glass interface, which has been discussed in detail in chapter 6 of this thesis, may change the wetting behavior of the glass. Alpha-Lactalbumin which is a pure protein showed strong effect on changing contact angle on its alteration toward more water-wet state (see chapter 7). This is likely due to surface modification by protein adsorption. Adsorption of -La to adsorb onto different solid surfaces has been reported by several authors (e.g. Halskau et al., 2002; Glomm et al., 2007).

Adsorption measurements by use of UV spectroscopy also showed adsorption of esterase enzymes on silica and kaolin (see chapter 7).

The interfacial tension between crude oil and brine containing enzyme or protein was measured as a function of equilibration time (see chapter 8). According to the results, interfacial tension between crude oil and enzyme-brine solutions was not significantly different from that between crude oil and brine. An exception was GZ which decreased the oil-brine IFT by a factor of approximately 3. GZ is a commercial enzyme which has been modified according to the crude oil complexity and is a compound of enzyme and stabilizers (surfactants) (Greenzyme material safety data sheets). The existence of stabilizers (surfactants) in the solution seems to make it exception among all tested enzymes and protein to be more interfacially active. Although esterase enzymes are interfacially active (Fojan et al., 2000; Al-Zuhair et al., 2007), it appears that their adsorbance on the crude oil-brine interface could not change the IFT. High concentration of interfacially active components in the crude oil can be the reason behind that. The affinity of the esterase enzymes for the interface seems to be not strong enough to replace the crude oil components. The result of IFT measurements using mineral oil and enzyme-brine solution which has been discussed further in section 8.1.3 shows good agreement with this argument. Adding enzyme to the brine solution could lower the IFT to some degree. It means that esterase enzymes are interfacially active when their substrate changes from crude oil to mineral oil